fix: adversarial mode survives a failed proposer (closes #9)

[ernestprovo23]

Summary

--mode adversarial had a single point of failure: when the lone proposer returned a malformed response (observed: gemini missing candidates[0].content.parts → KeyError), the proposal failed and the entire run aborted with "no verdict produced" — critics and judge never ran. This makes the adversarial flow resilient in three layered behaviors.

Closes #9.

Three behaviors implemented

1. Proposer fallback (run_adversarial): members are tried as proposer in council order, starting with the requested one. A member returning an unusable answer (ModelAnswer.error set / no text) is recorded and the next member is tried until one produces a usable proposal. The failed attempt is excluded from the critic set so a member is never both proposer and critic.

2. Graceful degrade (_degrade_to_synthesize): if no member can propose, the run degrades to a plain synthesize over the survivors and surfaces an actionable warning instead of silently emitting "no verdict produced". The warning is carried on CouncilResult.synthesis_error (the existing CouncilResult-level degrade signal, rendered yellow by the CLI) and mirrored to result.adversarial.verdict_error so field-specific consumers (CLI adversarial renderer, library callers) also see it. No new schema — reuses existing fields.

3. Adapter hardening (adapters/gemini.py): the adapter already converts a missing candidates[0].content.parts (and blocked/empty candidates) into a typed, redact-safe ProviderError, which call_model records as ModelAnswer.error — it never raises out of the run. Added regression tests pinning this at both the adapter unit level and end-to-end through call_model.

BYO-keys posture

No keys stored/logged/serialized. All error text flows through the existing ProviderError/redact() path. call_model's never-raise-for-member-failure invariant is preserved.

Tests

• test_adversarial_failed_proposer_falls_back_to_next_member — chosen proposer fails → next member used → real verdict produced.
• test_adversarial_explicit_failed_proposer_falls_back — explicit --proposer that fails still falls back.
• test_adversarial_all_proposers_fail_degrades_to_synthesize — all proposers fail → graceful degrade, warning surfaced on synthesis_error + mirrored to verdict_error, critics never run.
• test_gemini_parse_missing_content_parts_raises_provider_error (parametrized) — adapter unit level.
• test_call_model_gemini_missing_parts_becomes_error — end-to-end via call_model, proves .error set, no raise.

Local: 126 passed (was 121). ruff check . and ruff format --check . both clean.

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